Vehicle speed or cruise control systems typically include an actuator for positioning the throttle of the vehicle engine in relation to the difference between a commanded vehicle speed and a measure of the actual vehicle speed. Such difference, commonly referred to as the speed error, is periodically computed and used to develop a desired throttle position, and a feedback control system brings the actual throttle position into correspondence with the desired position.
The development of the desired throttle position generally involves some mathematical processing of the speed error and other related signals, and a calculation or table-look up that takes vehicle-specific characteristics and time constants into account. In general, the mathematical processing can be characterized as comprising various combinations of proportional, integral and derivative terms. For example, a PI or proportional-plus-integral control entails the summation of a first term proportional to the speed error itself, and a second term proportional to the integral of the speed error. The proportional term alone may be adequate under most conditions, but its gain is usually limited to ensure control system stability. In a cruise control systems, this means that small speed errors due to road grade, for example, may remain uncorrected. The integral term is used to correct such errors and thereby improve the control response to non-zero road grades because the value of the integral term increases with cumulative speed error. However, if the integral gain is set high enough to provide quick response to changing road grade, it tends to produce undulations in the commanded throttle position when the road grade is essentially constant. Such undulations are not only unnecessary, but also undesired, as they may be noticeable to the vehicle passengers and accelerate wear of the throttle actuator. Accordingly, what is needed is a vehicle cruise control that has the advantages of integral gain without the above-described disadvantages.